A common vegetable oil found in countless processed foods on supermarket shelves may be a significant, yet overlooked, driver of obesity, according to new scientific research. The study suggests the danger lies not in the oil itself, but in how our bodies process it when consumed in large quantities.
The Genetic Clue to Weight Gain Resistance
Scientists at the University of California, Riverside conducted experiments on mice to unravel why soybean oil seems more likely to cause weight gain than other fats. They fed two groups of mice a diet rich in soybean oil. The first group was genetically normal, while the second group was engineered to produce a slightly different form of a liver protein that influences fat metabolism.
The results were striking. The normal mice gained significant weight, but the genetically modified mice did not, despite consuming the same high-fat diet. This finding, published in late 2025, points to a crucial interaction between modern diets and individual genetics.
"This may be the first step toward understanding why some people gain weight more easily than others on a diet high in soybean oil," said Sonia Deol, a biomedical scientist at the university and the study's corresponding author.
From Linoleic Acid to Inflammation: The Metabolic Pathway
The research builds on a 2015 study from the same team that first identified soybean oil as more obesogenic than coconut oil. Professor Frances Sladek, a cell biology expert involved in both studies, clarified the new insight: "We've known... that soybean oil is more obesogenic... But now we have the clearest evidence yet that it’s not the oil itself, or even linoleic acid. It’s what the fat turns into inside the body."
The key lies in linoleic acid, a primary fatty acid in soybean oil. The body converts this acid into molecules called oxylipins. Excessive consumption leads to a surplus of these oxylipins, which are directly associated with increased inflammation and fat accumulation.
The genetically engineered mice in the study had significantly fewer oxylipins and showed healthier livers. They also exhibited enhanced mitochondrial function, which may help explain their resistance to weight gain. Analysis showed these mice had much lower levels of the enzymes that convert linoleic acid into oxylipins.
A Ubiquitous Ingredient with Long-Term Health Implications
Soybean oil is pervasive in the modern diet, though not typically used as a home cooking oil. It is a common ingredient in a vast array of processed items, including:
- Salad dressings and mayonnaise
- Margarine and many spreads
- Crisps and packaged snacks
- Ready meals and baked goods
Professor Sladek emphasised that the oil is not inherently bad, but the scale of consumption is problematic. "Soybean oil isn’t inherently evil," she stated. "But the quantities in which we consume it is triggering pathways our bodies didn’t evolve to handle."
While no human trials are currently planned, the researchers hope their work will guide future nutrition policy. Drawing a sobering historical parallel, Prof Sladek noted: "It took 100 years from the first observed link between chewing tobacco and cancer to get warning labels on cigarettes. We hope it won’t take that long for society to recognise the link between excessive soybean oil consumption and negative health effects."
The study underscores that obesity is a complex condition influenced by both diet and genetics. For consumers, it highlights the importance of scrutinising ingredients lists on processed foods and being mindful of the cumulative intake of common oils like soybean oil.
